Coating compositions for inorganic casting molds and cores, comprising formic acid esters, and use thereof

ABSTRACT

The subject matter of the invention relates to coating compositions comprising formic acid esters, an aqueous carrier liquid and pulverulent refractory material, and to the use thereof for casting molds, in particular those produced using water glass as a binder.

The invention relates to a sizing composition comprising formic acid esters as a coating composition for casting molds. The sizing composition is suitable for cores and molds producing water glass as the binder.

PRIOR ART

Casting molds can be produced from a refractory material, for example quartz sand, by shaping into a casting mold and binding using a suitable binder to guarantee adequate mechanical strength. Thus casting molds are produced using a refractory basic material and a suitable binder. The refractory basic mold material is preferably present in free-flowing form, so that it can be packed into a suitable hollow mold and compacted there. The binder produces firm cohesion between the particles of the basic mold material, so that the mold form achieves the necessary mechanical stability.

Both organic and inorganic binders can be used for producing casting molds, and they can be cured by cold or hot processes. Cold processes are those that are essentially performed at room temperature without heating the casting molds. In these cases the curing is usually performed by a chemical reaction, initiated, for example, by passing a gas as a catalyst through the mold to be cured. In hot processes, the molding mixture, after forming, can be heated to a temperature high enough, for example, to expel the solvent present in the binder or to initiate a chemical reaction by which the binder is cured, for example by cross-linking.

Regardless of the curing mechanism, all organic systems share the fact that they undergo thermal decomposition when the liquid metal is filled into the casting mold and in this process may release toxic materials, such as benzene, toluene, xylene, phenol, formaldehyde and other cracking products, some of which have not been identified.

It has become possible to minimize these emissions by various measures, but they cannot be avoided altogether in the case of organic binders. To minimize or prevent the emission of decomposition products during the casting process, binders may be used which are based on inorganic materials or which contain a very small fraction of organic compounds. Such binder systems have long been known.

Binder systems have been developed which can be cured by passing gases into them. A system of this type is described, for example, in GB 782205, in which an alkali water glass is used as the binder and can be cured by passing in CO₂. In U.S. Pat. No. 6,972,059 B1 an exothermic feeder mass containing an alkali silicate as binder is described. In addition, binder systems that are self-curing at room temperature have been developed. One such system, based on phosphoric acid and metal oxides, is described for example in U.S. Pat. No. 5,582,232. Finally, inorganic binder systems that are cured at higher temperatures, for example in a hot tool, are also known. Such hot-curing binder systems are known, for example, from U.S. Pat. No. 5,474,606, in which a binder system consisting of alkali water glass and aluminum silicate is described.

In U.S. Pat. No. 7,022,178 B1 a binder system for molding sands for producing cores is described. The binder system, based on water glass, consists of an aqueous alkali silicate solution and a hygroscopic base, for example sodium hydroxide, which is added in a ratio of 1:4 to 1:6. The water glass has a SiO₂/M₂O modulus of 2.5 to 3.5 and a solids fraction of 20 to 40%. To obtain a free-flowing molding material mixture that can also be packed into core molds with complicated shapes, and to control the hygroscopicity, the binder system also contains a surface-active material, such as silicone oil, which has a boiling point ≧250° C. The binder system is mixed with a suitable refractory material, such as quartz sand, and can then be shot into a core box using a core shooting machine. The curing of the molding material mixture is accomplished by withdrawing the remaining water. The drying or curing of the casting mold can also take place with the aid of microwaves.

The use of organic acids such as formic acid in sizes for the formation of metal salts with these acids is known, for example from GB 2294040 A and DE 2129479 A. Formic acid esters and their influence on the stability of the molds and cores are not disclose in GB 2294040 A and DE 2129479 A.

However, inorganic binders also have drawbacks compared with organic binders; for example, the known inorganic binders result in low stability of the casting molds produced by using them toward high atmospheric humidity and toward water. Thus storage of the moldings over a long period of time, as is customary with organic binders, is not definitely possible. To achieve higher initial strengths, better resistance of the casting molds toward atmospheric moisture and better results on the surface of the casting, in U.S. Pat. No. 7,770,629 B2 a molding mixture is suggested which in addition to a refractory basic mold material contains a water glass-based binder. A fraction of a particulate metal oxide is added to the mold material mixture. Preferably, precipitated silica or pyrogenic silica is used as the particulate metal oxide.

The methods described for producing casting molds and cores generally also include the application of a refractory mold coating, also known as sizing agent, at least to the surfaces of the basic mold that come into contact with the cast metal. The mold coatings are primarily intended to influence the surface of the molded parts, to improve the appearance of the casting, to influence the casting metallurgically, and/or to prevent casting defects.

The sizing agents generally used contain as their basic materials, for example, clays, quartz, diatomaceous earth, crystobalite, tridymite, aluminum silicate, zirconium silicate, mica and/or fireclay and also coke and graphite. These basic materials are the defining parts of the sizing agent, which cover the mold surface and close the pores against the penetration of the casting metal.

With the aid of these coatings, therefore, the surface of the casting mold can be modified and tailored to the properties of the metal being processed. For example, the sizing agent can improve the appearance of the casting by producing a smooth surface, since the sizing agent smooths out irregularities caused by the [different] sizes of the granules of the molding material. In addition, the sizing agent can metallurgically influence the casting in that, for example, through the sizing agent, additives at the surface of the casting are selectively transferred into the casting, improving the surface properties of the casting.

In addition, the sizing agents form a layer that chemically isolates the casting mold during the casting of liquid metal. In this way, any adhesion between the casting and the casting mold is prevented, so that the casting can be removed from the casting mold without problems. However, the sizing agent can also be used to control the heat transfer between liquid metal and casting mold, in order for example to accomplish the formation of a certain metal microstructure via the cooling rate.

Today, the curing of the inorganic binders used takes place increasingly via a condensation reaction initiated by elevated temperatures, in which the formation of the binder bridges occurs through splitting off of water. Like many chemical reactions, this is a reversible reaction, i.e., the bonds can be broken by contact and reaction with water, wherein the extent of this reaction depends heavily on the process parameters for core production. Under the process parameters typically used in mass production (rapid cycling, high temperatures), the casting molds lose their strength due to contact with water and to some extent also, to contact with alcohol; the surface softens and the casting mold loses its shape.

STATEMENT OF THE PROBLEM

The invention was based on the task of suggesting a sizing agent through which the most defect-free possible coating, especially of inorganic cores and molds, can be assured, without negatively impacting the stability of the cores or molds and thus their processing and storage.

SUMMARY OF THE INVENTION

This task is accomplished with the features of claim 1; advantageous embodiments form the subject matter of the dependent claims or are described in the following.

In a preferred embodiment, the sizing composition according to the invention is prepared in the form of a paste or a suspension. In this embodiment, the sizing composition contains a carrier fluid.

Surprisingly it was found that when certain additives are added to an aqueous sizing composition, the quality of the dressed inorganic cores and molds can be lastingly improved, and for example, a shelf life of several days for the cores and molds can be achieved without difficulty.

DETAILED DESCRIPTION OF THE INVENTION

Esters of formic acid (methanoic acid) are used as an additive component of the sizing composition, wherein the mean chain length of the alcohol or alcohol mixture used in the esterification is especially less than 6 carbon atoms and particularly preferably less than 3 carbon atoms. Formic acid methyl ester (methyl formate) and formic acid ethyl ester (ethyl formate) are particularly preferably used. It is also possible for the alcohol group(s) or some of the alcohol groups to bear one or two additional groups such as ether groups, hydroxy groups, ester groups or carboxyl groups, or for the formic acid to be cross-linked through the second or third hydroxy group, e.g., by condensation.

The total quantity of the above additives, based on the sizing composition, amounts to 1 to 8%, preferably 2 to 8%, and particularly preferably 3 to 6%. For example, “methyl formate pure” from BASF may be used as the additive. It has the CAS number 107-31-3.

Depending on the desired use of the sizing composition, e.g., as a base coat or top coat, and the desired layer thickness of the coating produced from the sizing composition, additional characteristic parameters of the sizing composition can be set.

The carrier fluid may be partially or completely composed of water. The carrier fluid is the component that can be evaporated at 160° C. and normal pressure and in this sense, by definition, is primarily the component that is not the solids content. The carrier fluid contains water to the extent of more than 50% by weight, preferably 75% by weight, especially more than 80% by weight, and optionally greater than 95% by weight of water.

The additional constituents in the carrier fluid may be organic solvents. Suitable solvents are alcohols, including polyalcohols and polyether alcohols. Examples of alcohols are ethanol, n-propanol, isopropanol, butanol and glycol.

The solids content of the ready-to-use sizing composition is preferably adjusted in the range of 10 to 85-% by weight, or in the sale-ready form (before dilution), especially 30 to 70-% by weight.

For example, the sizing agent can comprise the combination of certain clays as constituents of the sizing agents. The clay material used was a combination of

-   -   a) 1 to 10 parts by weight, especially 1 to 5 parts by weight,         of palygorskite,     -   b) 1 to 10 parts by weight, especially 1 to 5 parts by weight,         of hectorite and     -   c) 1 to 20 parts by weight, especially 1 to 5 parts by weight,         of sodium bentonite         (in each case relative to one another), especially in a weight         ratio of palygorskite to hectorite of 1 to 0.8 through 1.2 and a         ratio of palygorskite to hectorite together to sodium bentonite         of 1 to 0.8 through 1.2. The total clay content of the above         clays in the sizing agents is 0.1 to 4.0% by weight, preferably         0.5 to 3.0% by weight and particularly preferably 1.0 to 2.0% by         weight, based on the solids content of the sizing composition.         The shelf life of the cores and molds can be further increased         using these clays.

In addition, the sizing composition can contain salts of the metals magnesium and/or manganese in a concentration of, optionally in combination, greater than 1% by weight, based on the sizing composition. Particularly suitable for use as anions of the salts are sulfate ion{s} and or chloride ions, wherein the magnesium is especially used in the +2 or +4 oxidation state and/or manganese in the +2 oxidation state and the concentration of the salts in particular amounts to more than 3-% by weight and especially greater than 5-% by weight, based on the sizing composition. According to one embodiment, the concentration of the salts amounts to maximally the saturation concentration of the carrier fluid used or the concentration amounts to less than 10-%, but in that case based on the sizing composition.

Thus the sizing composition according to the invention contains at least one powdered refractory material. This refractory material serves to block the pores in a casting mold against the penetration of the liquid metal. In addition, the refractory material achieves thermal insulation between the casting mold and the liquid metal. Examples of suitable refractory materials are quartz, aluminum oxide, zirconium oxide, aluminum silicates such as pyropillite, kyanite, andalusite or fireclay, zirconium sands, zirconium silicates, olivine, talc, mica, coke, feldspar, diatomite, kaolins, calcined kaolins, kaolinite, metakaolinite, iron oxide and/or bauxite.

The refractory material is prepared in powdered form. The particle size is selected such that a stable structure is formed in the coating and that the sizing agent preferably can be distributed without problems on the wall of the casting mold using a spray device. Suitably, the refractory material has a mean particle size (measured by light scattering according to DIN/ISO 13320) in the range of 0.1 to 500 μm, particularly preferably in the range of 1 to 200 μm. Particularly suitable as refractory materials are those that have a melting point that is at least 200° C. above the temperature of the liquid metal and independently of this, do not react with the metal.

The fraction of the refractory material (in each case contributing only to the solids content), e.g., in the commercial form of a paste, relative to the solids fraction of the sizing material composition, is advantageously more than 70% by weight, preferably more than 80% by weight, and particularly preferably more than 85% by weight.

According to one embodiment (in each case contributing only to the solids content), the fraction of the refractory material is selected to be less than 70-% by weight, according to another embodiment less than 60-% by weight and according to another embodiment less than 50-% by weight.

In another embodiment the sizing material according to the invention may contain at least one suspending agent. The suspending agent brings about an increase in the viscosity of the sizing agent, so that the solid constituents of the sizing agent do not drop to the bottom in the suspension, or do so to only a slight degree. Organic or inorganic materials or mixtures thereof can be used to increase the viscosity. Suitable inorganic suspending agents are, for example, strongly swelling clays, such as sodium bentonite.

Alternatively or additionally, the suspending agent may also include organic thickeners, since after application of the protective coating, these can be dried to such an extent that they scarcely release any water after application of the protective coating. Possible organic suspending agents are, for example, swellable polymers such as carboxymethyl-, methyl-, ethyl-, hydroxyethyl- and hydroxypropylcelluloses, plant mucilages, polyvinyl alcohols, polyvinylpyrrolidone, pectin, gelatin, agar-agar, polypeptides and/or alginates.

The fraction of the suspending agent, based on the total sizing composition, is advantageously selected as 0.1 to 5% by weight, preferably 0.5 to 3% by weight, particularly preferably 1 to 2% by weight.

According to a preferred embodiment, the sizing agent according to the invention contains at least one binder as an additional component. The binder makes possible better fixation of the sizing agent or the protective coating made from the sizing agent onto the wall of the casting mold. In addition, the binder increases the mechanical stability of the protective coating, so that less erosion is observed under the influence of the liquid metal. Preferably the binder cures irreversibly, so that an abrasion-resistant coating is obtained. Particularly preferred are binders that do not soften again under contact with atmospheric humidity. Fundamentally any binder used in sizing agents may be present. Both inorganic and organic binders may be used. Suitable binders for use include, for example, clays, especially bentonite and/or kaolin.

The fraction of the binder is preferably selected in the range of 0.1 to 20% by weight, particularly preferably 0.5 to 5% by weight, based on the solids content of the sizing composition.

According to an additional, preferred embodiment, the sizing agent contains a graphite fraction. This supports the formation of lamellar carbon at the interface between casting and casting mold. The fraction of the graphite is preferably selected in the range of 1 to 30% by weight, especially preferably 5 to 15% by weight, based on the solids content of the sizing composition. In iron casting, graphite has a beneficial effect on the surface quality of the casting.

Optionally, the sizing composition according to the invention may contain other constituents customary in sizing agents, for example wetting agents, defoamers, pigments, colorants or biocides. The fraction of these additional constituents in the ready-to-use coating composition is advantageously less than 10% by weight, preferably less than 5% by weight and particularly preferably less than 1% by weight.

For example, wetting agents may include anionic and non-anionic surfactants, especially those with an HSB value of at least 7. One example of such a wetting agent is disodium dioctylsulfosuccinate. The wetting agent is preferably used in a quantity of 0.01 to 1% by weight, preferably 0.05 to 0.3% by weight, based on the ready-to-use sizing composition.

Defoamers, also known as antifoamers, can be used to prevent foam formation during the production or application of the sizing composition. Foam formation during application of the sizing composition can lead to lack of uniformity in the layer thickness and to holes in the coating. For example, silicone oil or mineral oil may be used as defoamers. The defoamer is advantageously present in a quantity of 0.01 to 1% by weight, preferably of 0.05 to 3% by weight, based on the ready-to-use sizing composition.

In the sizing composition according to the invention, the usual pigments and colorants may be used if desired. These are added to achieve contrast, e.g., between different sizing agents, or to create a stronger separating effect of the sizing agent from the casting.

Examples of pigments are red and yellow iron oxide and graphite. Examples of colorants are commercial colorants such as the Luconyl® colorant series from BASF AG, Ludwigshafen, DE. The colorants and pigments are preferably present in a quantity of 0.01 to 10% by weight, preferably of 0.1 to 5% by weight, based on the solids content of the sizing composition.

According to an additional embodiment, the sizing composition contains a biocide to prevent bacterial attack and thus avoid a negative effect on the rheology and the binding strength of the binder.

This is especially preferred when the carrier fluid contained in the sizing composition is made up essentially (on a weight basis) of water, and thus the sizing agent according to the invention is supplied in the form of a so-called aqueous sizing agent. Examples of suitable biocides are formaldehyde, 2-ethyl-4-isothiazolin-3-one (MIT), 5-chloro-2-methyl-4-isothiazolin-3-one (CIT) and 1,2-benzisothiazolin-3-one (BIT). Preferably, MIT, BIT or a mixture thereof is used. The biocides are usually used in a quantity of 10 to 1000 ppm, preferably of 50 to 500 ppm, based on the weight of the ready-to-use sizing composition.

In addition to the constituents mentioned in the preceding, the sizing composition according to the invention may contain additional constituents customary for sizing agents.

The sizing composition according to the invention may be made by the usual methods. For example, a sizing composition according to the invention can be prepared by initially taking water and then dispersing clay, acting as a suspension agent, in it using a high-shear agitator. Then the refractory components, pigments and colorants, as well as the metal additive are mixed in until a homogeneous mixture forms. Finally, wetting agents, antifoamers, biocides and binders are mixed in.

The sizing composition according to the invention can be prepared and sold as a ready-to-use formulated sizing agent. However, the sizing agent can also be prepared and sold in concentrated form. In this case, to produce a ready-to-use sizing agent, the quantity of the carrier fluid needed to establish the desired viscosity and density of the sizing agent is added. In addition, the sizing agent according to the invention can also be prepared and sold in the form of a kit, wherein for example the solid component(s) and the solvent component(s) are present side by side in separate containers. The solid component(s) can be supplied as a powdered solids mixture in a separate container. Additional liquid component(s) optionally to be used, for example binders, surfactants, wetting agents/defoamers, pigments, colorants and biocides, can also be present in this kit, once again in a separate container. The solvent component(s) can either include the optional additional components for use, e.g., in a single container, or they can be present separate from other optional components in a separate container.

To produce a ready-to-use sizing agent, the suitable quantities of the solids components, the optional additional components and the solvent components are mixed together.

The sizing compositions according to the invention are suitable for coating casting molds. The term “casting mold” used here includes all types of solid bodies needed for producing a casting, e.g., cores, molds and dies. The use of the sizing agent according to the invention also includes partial coating of casting molds. The sizing agents are used for casting molds for metal processing, obtainable from inorganic molding material mixtures containing at least one refractory basic mold material, a water glass-based binder and preferably a fraction of a metal oxide selected from the group of silicon dioxide, especially amorphous silicon dioxide, aluminum oxide, titanium oxide or zinc oxide and mixtures thereof, wherein this is preferably present in particulate form and especially has particle sizes of less than 300 μm (screen analysis). For example, amorphous silicon dioxide can be obtained through precipitation processes starting from water glass, which can be obtained by solubilizing quartz said with sodium carbonate or potassium carbonate. Depending on the process conditions, SiO₂ obtained in this way is called precipitated silica. Another important manufacturing variant is the production of so-called pyrogenic SiO₂ in an oxyhydrogen gas flame, starting from liquid chlorosilanes such as silicon tetrachloride.

The molding material mixtures and a method for producing casting molds for metal processing using the cured molding material mixtures are described in WO 2006/024540 (+U.S. Pat. No. 7,770,629 B2), and this publication is incorporated in the content of this application to this extent. Preferred molding material mixtures are the subject matter of the claims of WO 2006/024540. Thus the casting molds to be coated typically comprise

-   -   a) More than 80% by weight of a refractory basic mold material         (including additives that behave like refractory basic mold         materials).     -   b) 0.01 to 5% by weight of cured water glass as a binder,     -   c) optionally 0 to 5% by weight of the above-named metal oxide         or metal oxides.

In addition, the invention relates to a method for producing sized casting molds for metal processing with application of sizing agents to the above-mentioned, partially or completely cured inorganic molding material mixtures. The application or preparation of the sizing agents can take place as follows:

In the case of dipping as an application method, the casting mold, in the mold cavity of which a base coat of sizing agent has been applied, is dipped for about 2 seconds to 2 minutes in a container that is filled with a ready-to-use sizing composition according to the invention.

The casting mold is then removed from the sizing composition and excess sizing composition is allowed to run out of the casting mold. The time taken for the excess sizing composition to run out after dipping depends on the run-off behavior of the sizing composition used.

In the case of spraying as the application method, commercial pressure tank spraying devices are used. In this process the sizing composition, in a diluted state, is filled into a spray tank. Through the positive pressure to be applied, the sizing agent can be forced into a spray gun, where it is sprayed with the aid of separately controllable atomizing air. During spraying, the conditions are advantageously selected such that the pressure for the sizing composition and atomizing air are selected on the spray gun is set so that the sizing composition that is sprayed atomizing air strikes the mold or the core while said sizing composition is still wet, but results in a uniform application.

In the case of flow coating as the application method, the casting mold, optionally with a base coat applied in the mold cavity, is covered with a ready-to-use sizing composition with the aid of a tube, a lance or similar instruments.

In this process, the casting mold is covered completely with the sizing composition and the excess sizing composition runs out of the casting mold. The time taken for the excess sizing composition to run off after flow coating depends on the flow behavior of the sizing composition used.

In addition, the sizing agent can also be applied by painting.

The carrier fluid contained in the sizing agent is then evaporated, so that a dry sizing agent layer is obtained. Drying methods used may include all conventional drying methods, for example allowing to dry in air, drying with dehumidified air, drying with microwave or infrared radiation, drying in a convection oven and comparable methods.

In a preferred embodiment of the invention, the coated casting mold is dried in a convection oven at 20 to 250° C., preferably 50 to 180° C.

When alcohol-based sizing agents are used, the sizing composition according to the invention is preferably dried by burning off the alcohol or the alcohol mixture. In this case the coated casting mold is additionally heated by the heat of combustion. In an additional preferred embodiment, the coated casting mold is dried in air without further treatment or using microwaves.

The sizing agent can be applied in the form of a single layer or in the form of several layers arranged one on top of the other. In this process the individual layers may be the same or different in terms of their composition. For example, first a base coat can be produced from a conventional commercial sizing agent that contains no metal additive according to the invention. For example, water-based sizing agents or alcohol-based sixing agents may be used. It is also possible to prepare all layers from the sizing composition according to the invention. The layer that will later come into contact with the liquid metal, however, is always produced from the sizing agent according to the invention. When several layers are applied, each individual layer may be dried completely or partially after application.

The coating prepared from the sizing composition advantageously has a dry layer thickness of at least 0.1 mm, preferably at least 0.2 mm, particularly preferably at least 0.45 mm, especially preferably at least 0.55 mm. According to one embodiment, the thickness of the coating is selected to be less than 1.5 mm. The thickness of the dry layer in this case is the layer thickness of the dried coating obtained by essentially completely removing the solvent component and optionally curing subsequently. The dry layer thickness of the base coat and the top coat are preferably determined by measuring with the wet layer thickness comb. The casting mold can then be assembled completely if desired. The casing is advantageously performed to make iron and steel castings.

Example

In this example the effects of additional various types of waterglass hardeners in sizing agents on the strengths of the sized cores were examined. Core layers 1 to 3 have the compositions indicated in Tables 1 to 3.

TABLE 1 Composition of the sizing agents Sizing Sizing Sizing agent 1 agent 2 agent 3 % by % by % by Component weight weight weight Water 28.5 28.5 32.5 Methyl formate 4 — — Ethyl formate — 4 — Zircon flour 45 μm 22.5 22.5 22.5 Attagel (attapulgite) 40 1.5 1.5 1.5 PVA solution (25%) 1.5 1.5 1.5 Satintone W ® 17 17 17 Surfynol ® SEF (wetting agent 1) 0.2 0.2 0.2 Dynol ® (wetting agent 2) 0.05 0.05 0.05 Foamstar ® MF342 (defoamer) 0.2 0.2 0.2 Pyrax RG 140 12 12 12 Gloss graphite powder 85% C 12 12 12 Acticide ® F(N) (preservative 1) 0.4 0.4 0.4 Acticide ® MBS (preservative 2) 0.15 0.15 0.15

The mold casting sizing agent was produced as follows: water is placed in a vessel and the clay(s) were broken up using a high-shear unit and a chopper disk for at least 15 minutes. Then the refractory components, pigments and colorants are mixed in for at least 15 minutes until a homogeneous mixture forms. Together with the solids described the additives methyl formate and ethyl formate were added to sizing agent 1 and sizing agent 2. Finally, additives such as wetting agents, defoamers and preservatives as well as the binder are mixed in for 5 minutes.

For the experiments that follow, the sizing agents were adjusted to a viscosity in the range of 0.6 Pas, suitable for application. The adjustment was done by adding appropriate amounts of water to the original composition and then homogenizing. The main characteristic in this case is the viscosity, measured at 20° C. with a Brookfield viscometer (DIN EN ISO 2555) and with a DIN 4 mm viscosity flow cup (DIN EN ISO 2431).

TABLE 2 Application parameters Sizing Sizing Sizing agent 1 agent 2 agent 3 Brookfield [Pas] 0.6 0.6 0.6 Flow time [s] 15.5 15.5 15.5 Solids content [%] 66.9 66.9 66.9 Penetration depth 2 2 2 [mm] according according not to patent to patent according to patent

The methyl formate contained in sizing agent 1, along with other characteristics, is able to undergo reaction with the inorganic binder that is partially dissolved during the sizing process. It produces complete curing of the dissolved inorganic binder and thus greater stability of the sized cores. Conversely, reducing the content in the layer leads to a loss of strength in the core. The same is true for the ethyl formate used in sizing agent 2. In the following, the performance of the corresponding series of experiments, which served as the basis for the observations described, will be described in detail.

Comparable behavior can likewise be achieved with chemically related esters.

Producing and Testing the Molding Materials Test Pieces

Two different core geometries were investigated for experimental purposes. One geometry, the so-called Georg Fischer test bar, illustrates the behavior of the sized test pieces on thicker core geometries, while the other geometry, so-called long cores, illustrates the behavior of the sized test pieces on thin geometries. Georg Fischer test bars are defined as rectangular test bars with dimensions of 150 mm×22.36 mm×22.36 mm. The long cores have dimensions of 13 mm×20 mm×235 mm.

The composition of the molding material mixture is shown in Table 3. The following procedure was used for producing the Georg Fischer test bar: the components listed in Table 3 were mixed in a laboratory paddle vane type mixer (from Vogel & Schemmann AG, Hagen DE).

For this purpose, first the quartz sand was placed in a vessel and the water glass was added while stirring. A sodium water glass containing some potassium was used as the water glass. The SiO₂:M₂O modulus of the water glass was about 2.2, wherein M is the sum of sodium and potassium. After the mixture was agitated for one minute, amorphous silicon dioxide was optionally added under further agitation. The amorphous silicon dioxide is pyrogenic silica from the RW Silicium company. Then the mixture was agitated for an additional minute.

The molding composition mixture was transferred to the storage bunker, an H 2.5 Hot Box core shooting machine from the Röperwerk-Giessereimaschinen GmbH, Viersen, DE, the molding tool of which was heated to 180° C. The molding composition mixture was introduced into the molding tool using compressed air (5 bar) and remained in the molding tool for an additional 35 seconds. To accelerate the curing of the mixture, hot air (2 bar, 150° C. upon entry into the tool) was passed through the molding tool for the last 20 seconds. The molding tool was opened and the test bar removed.

The coating compositions were applied to the test bar by dipping; the application parameters are listed in Table 2. The test bars were coated either immediately after removal from the molding tool or after a 30 min cooling time. After application of the coating, the coated test bars were held in a drying oven for 30 min at 150° C.

To determine the bending strengths, the test bars were placed in a Georg Fischer strength testing apparatus equipped with a 3-point bending device (DISA Industrie AG, Schaffhausen, CH) and the force resulting in breakage of the test bar was measured.

The bending strengths of the uncoated test bars were measured according to the following plan:

-   -   10 seconds after removal (hot strength)     -   1 hour after removal (cold strength).         -   The test bars were coated according to the following scheme:     -   coated 1 minute after removal (hot coating)     -   coated 30 minutes after removal (cold coating)         -   The coated bending strengths of the coated test bars were             measured according to the following plan:     -   10 seconds after removal from the drying oven         -   (hot strengths, hot-coated)     -   coated 30 minutes after removal         -   (cold strengths, hot-coated, immediately after removal from             oven)     -   10 seconds after removal from the drying oven         -   (hot strengths, cold-coated, room temperature 20° C.     -   coated 30 minutes after removal         -   (cold strengths, cold-coated)

TABLE 3 Composition of molding composition mixtures Quartz Alkali sand water Amorphous H32 glass silica Coating 1 100 GT 2.0 0.5 — Comparison, not according to invention 2 100 GT 2.0 0.5 Sizing According to agent 1 invention 3 100 GT 2.0 0.5 Sizing According to agent 2 invention 4 100 GT 2.0 0.5 Sizing Not according agent 3 to invention

TABLE 4 Strengths in Georg Fischer test bars Hot Cold Hot Cold strength, strength, strength, strength, hot hot cold cold coated coated coated coated [N/cm2] [N/cm2] [N/cm2] [N/cm2] 1 400 450 400 450 Comparison, not according to invention 2 300 400 320 470 According to invention 3 300 380 320 450 According to invention 4 250 — — 340 Not according (broken) (broken) to invention

TABLE 5 Strengths on long cores Hot Cold Hot Cold strength, strength, strength, strength, hot hot cold cold coated coated coated coated [N/cm2] [N/cm2] [N/cm2] [N/cm2] 1 250 280 250 280 Comparison, not according to invention 2 200 230 240 260 According to invention 3 180 210 200 230 According to invention 4 — — — — Not according (broken) (broken) (broken) (broken) to invention

Results

The use of an alcohol-containing coating is undesirable because of the drawback presented by the release of emissions during the casting process. Using standardized water-based coating agents, the test pieces made of water glass-containing molding material agents lose considerable strength (sizing agent 4).

Surprisingly it was found, while seeking possibilities for suppressing the reaction between the water from the coating and the binder of the molding material, that the addition of formic acid esters to the coating distinctly suppresses the effect. The strengths are maintained. Esters with different chain lengths and structures can be used, as is done for example in sizing agents 1 and 2 in Example 1. The high strengths compared to those with cores sized with sizing agent 3 provide evidence for the positive effect. 

1. A sizing composition comprising: (A) esters of formic acid; (B) a carrier fluid comprising or consisting of water, and (C) refractory materials.
 2. The sizing composition according to claim 1, wherein the alcohol group(s) of the formic acid ester(s) in each case contain fewer than 10 carbon atoms.
 3. The sizing composition according to claim 2, wherein the concentration of the formic acid esters in the sizing composition amounts to 1 to 8% by weight.
 4. The sizing composition according to claim 1, wherein the carrier fluid comprises of more than 50% by weight of water.
 5. The sizing composition according to claim 1, wherein the solids content of the sizing composition amounts to 30 to 70% by weight.
 6. The sizing Sizing composition according to claim 1, wherein the sizing composition comprises 10 to 85% by weight of refractory material, based on the solids content of the sizing composition.
 7. The sizing composition according to claim 1, wherein the refractory materials are selected from the group consisting of: quartz, aluminum oxide, zirconium dioxide, aluminum silicates, zirconium sands, zirconium silicates, olivine, talc, mica, coke, feldspar, diatomite, kaolins, calcined kaolins, kaolinite, metakaolinite, iron oxide, bauxite and mixtures thereof.
 8. The sizing composition according to claim 1, in which the refractory materials have particle sizes of 0.1 to 500 μm, measured by light scattering according to DIN/ISO
 13320. 9. The sizing composition according to claim 1, wherein the sizing composition comprises 0.1 to 20% by weight, of at least one binder, based on the solids content of the sizing composition.
 10. The sizing composition according to claim 1, wherein the sizing composition comprises a lustrous coal forming agent or graphite. 11-14. (canceled)
 15. The sizing composition according to claim 1, wherein the alcohol group(s) of the formic acid ester(s) in each case contain fewer than 6 carbon atoms.
 16. The sizing composition according to claim 1, wherein the alcohol group(s) of the formic acid ester(s) in each case contain fewer than 3 carbon atoms.
 17. The sizing composition according to claim 2, wherein the concentration of the formic acid esters in the sizing composition amounts to 2 to 8% by weight.
 18. The sizing composition according to claim 2, wherein the concentration of the formic acid esters in the sizing composition amounts to 3 to 6% by weight.
 19. The sizing composition according to claim 4, wherein the carrier fluid further comprises alcohols, including polyalcohols and polyether alcohols.
 20. The sizing composition according to claim 19, wherein the carrier fluid can be fully evaporated at up to 160° C. and 1013 mbar.
 21. The sizing composition according to claim 1, in which the refractory materials have particle sizes of 1 to 200 μm, measured by light scattering according to DIN/ISO
 13320. 22. The sizing composition according to claim 1, wherein the sizing composition comprises 0.5 to 5-% by weight of at least one binder, based on the solids content of the sizing composition.
 23. The sizing composition according to claim 10, wherein the lustrous coal forming agent or graphite is present in the amount of 0.1 to 10% by weight.
 24. The sizing composition according to claim 10, wherein the lustrous coal forming agent or graphite is present in the amount of 0.5 to 3% by weight.
 25. A mold or core for casting, comprising: a molding material mixture, coated with a sizing composition according to claim 1; and a binder comprising water glass.
 26. The mold or core of claim 25, wherein: the molding material mixture comprises at least one member selected from the group consisting of: silicon dioxide, aluminum oxide, titanium oxide and zinc oxide.
 27. The mold or core of claim 25, wherein: the sizing composition is diluted with at least water before coating the molding material mixture.
 28. A method for sizing a mold or core for casting, comprising the steps of: providing a mold or core for casting, the mold or core comprising a molding material mixture and a binder comprising water glass; and coating the mold or core with a sizing composition comprising: esters of formic acid; a carrier fluid comprising water, and refractory materials.
 29. The method of claim 29, wherein: the molding material mixture comprises at least one member selected from the group consisting of: silicon dioxide, aluminum oxide, titanium oxide and zinc oxide.
 30. The method of claim 29, wherein: the sizing composition is diluted with at least water before the coating step. 